The invention relates to a method and apparatus for detecting slow and small changes of electrical signals including the sign of the changes, and circuit arrangement for the exact detection of the peak value of an alternating voltage. Under the term xe2x80x9celectrical signalsxe2x80x9d a direct current voltage or quantities that can be represented by the measurement of direct current voltages are understood, such quantities are e.g. output signals of current or temperature probes.
Direct current voltages can generally be measured with a required accuracy. There are, however, special tasks of measurements, wherein changes should be exactly detected, which are very low relative to the level of the direct current voltage, e.g. 10xe2x88x923 or 10xe2x88x924 times of the DC level, and such changes take place slowly, e.g. during a couple of hours. The difficulty of the task increases if the occurrence of such slow changes should be detected very fast that means less than a couple of minutes, and the detection time might be in the order of magnitude of 10 seconds. In case of such detection tasks conventional methods of measuring voltages cannot be used, since the useful signal is not higher than the accuracy of the measurements.
Typically such a task is the determination of the end of charge moment in case of charging batteries. Especially, when the battery is charged intensively with a high charging current, the charging process should be finished as soon as the fully charged state has been reached, otherwise the battery might suffer an irreversible damage. Thew end of charge state is often indicated by a very low change of the battery voltage which can be below 1 mV, or such an indication can be the end of a similarly low decrease of the battery voltage.
In the booklet of Motorola Inc. SG 73/D Rev. 17, 1998 of the Master Selection Guide series, an integrated battery charger circuit type MC 33340P is described that can detect the decrease of the battery voltage by a sensitivity of 4 mV. The required accuracy is much higher than this value, and it is not sufficient to detect the decrease of the voltage only, one has to determine the tendency of the change as well. The tendency means the determination whether the signal has decreased by a predetermined extent, it has increased at least by that extent or it has remained unchanged i.e. the fluctuations have not exceeded the predetermined level.
U.S. Pat. No. 4,137,493 describes a detector circuit for detecting changes in the level of a DC voltage used for controlling the end of charge moment of a battery charger. In this detector a capacitor is charged in sampling periods to the DC voltage, and in each sampling period if the DC voltage level has changed since the previous period, a charging or discharging current will flow through the lead out wire of the capacitor until it takes the new DC value. This transient current is monitored and compared with a reference voltage. The accuracy of this detector is limited by the non-compensated DC offset of the applied circuitry.
In case of very small changes of voltage signals there is no kind of reliable and accurate means available that would be able to detect the steepness of the changes or the persistence of an unchanged state of the signal. The knowledge of such parameters would be, however, desirable in several fields of the technique.
In case if the signal to be monitored is not constituted by a direct current voltage but by a quantity that is repeated periodically like pulses, then the detection problem will be more difficult, since no peak detector is known that could generate a direct current level from the pulsating electrical signal with the required accuracy. The non-linear components used for the detection have temperature-dependent properties which often fluctuate, and the direct current signal processing has both offset and drift errors. These side effects will not be negligible any more if such changes of the signal have to be determined, which are by orders of magnitude below the signal level.
A peak detector is described in DD patent 101 988, wherein the input AC voltage is rectified and filtered. This DC voltage is then sampled and a capacitor is charged. The sampling pulses are generated by forming the first and second quotient signal of the rectified voltage, and the circuit is capable of detecting a single maximum at a time. The accuracy is decreased by the short nature of the sampling pulses and by the fact that the rectified voltage has a substantial DC component that makes processing difficult.
The object of the invention is to provide a method and an apparatus that makes possible both the safe detection of the slow and small changes of a direct current voltage and the determination of the tendency of the changes, wherein the changes are by three decimal orders of magnitude smaller than the DC level, and which has a circuit design that facilitates mass production.
A further object of the invention is to provide a circuit arrangement that can carry out the peak detection of repetitive pulse signals without any fluctuation of the DC level and which has the required accuracy.
These objects have been met by the solution as defined in the attached claims.